This invention relates to a method of driving an active matrix type liquid crystal display unit.
Research and development of active matrix type liquid crystal display units in various fields have been conducted for the purpose of applying the technology to thin or flat television sets and the like.
An equivalent circuit diagram of the foregoing active matrix type liquid crystal display unit will now be described with respect to FIG. 3. In this drawing, SW.sub.i,SW.sub.i+1 designate switching elements made of transistors or the like, which are selected (turned on) and non-selected (turned off) in accordance with signals sent to scanning signal lines XL.sub.i,XL.sub.i+1. YL.sub.j,YL.sub.j+1 designate picture signal lines for supplying picture signals to pixel electrodes PX.sub.i,PX.sub.i+1 connected with the selected switching elements SW.sub.i,SW.sub.i+1. LC.sub.i,LC.sub.i+1 designate liquid crystal layers corresponding to individual pixels which are sandwiched by the pixel electrodes PX.sub.i,PX.sub.i+1 and a common electrode COMON. ST.sub.i,ST.sub.i+1 designate stick capacitors connected with the pixel electrodes PX.sub.i,PX.sub.i+1, which are provided for holding individual voltages supplied from the picture signal lines YL.sub.j,YL.sub.j+1. STACK designates a stick capacitor electrode for the stick capacitors ST.sub.i,ST.sub.i+1.
FIG. 9 is a time chart showing a method of driving the active matrix type liquid crystal display unit shown in FIG. 3. In this drawing, X.sub.i,X.sub.i+1 designate scanning signals applied to the scanning signal lines XL.sub.i,XL.sub.i+1, with the value of logic level "1" indicating "selection" and with the value of logic level "0" indicating "non-selection." Specifically, a selection signal of logic level "1" is supplied during a horizontal interval T.sub.H per vertical interval T.sub.V. Y.sub.j designates a picture signal applied to the picture signal line YL.sub.j, whose polarity is inverted about a reference voltage V.sub.C per vertical interval T.sub.V. This alternating-current drive mode is adopted for the purpose of preventing direct current from being applied to the liquid crystal. COM designates a common voltage applied to the common electrode COMON, which is always maintained at the reference voltage V.sub.C. PXL designates a pixel voltage applied to the pixel electrode PX.sub.i. In this connection, the stick capacitor ST.sub.i holds the value of the picture signal Y.sub.j supplied to the pixel electrode PX.sub.i when the switching element SW.sub.i is selected, even when the switching element SW.sub.i is brought into the "non-selection" mode. PXL-COM designates the signal of the pixel voltage PXL minus the common voltage COM, i.e., the voltage applied to the liquid crystal layer LC.sub.i, which has the same waveform as that of the pixel voltage PXL because the common voltage has a constant value V.sub.C. It should be noted that the voltage applied to the stick capacitor electrode STACK has the constant value V.sub.C.
FIG. 10 shows a light transmittance characteristic of the liquid crystal obtained in accordance with the foregoing driving method. In this drawing, the abscissa represents the effective voltage V.sub.LC applied to the liquid crystal layer and the picture signal voltage V.sub.SIG, whereas the ordinate represents the liquid crystal light transmittance T. According to the foregoing driving method, as described above, the voltage (PXL-COM) applied to the liquid crystal layer has a constant value V.sub.SIG because the common voltage COM is constant. Thus, its effective voltage is also "V.sub.SIG ". Therefore, the effective voltage V.sub.LC applied to the liquid crystal layer is identical with the picture signal voltage V.sub.SIG.
However, with such an active matrix type liquid crystal display unit, a gradational display is made by segmenting the span from 100% (white) to 0% (black) of the liquid crystal light transmittance T. Practically, the gradational display is attained by dividing the picture signal voltage V.sub.SIG so as to correspond to discrete values of light transmittance. Therefore, to obtain a fine gradational display, the voltage width of the picture signal voltage V.sub.SIG corresponding to where the light transmittance T varies from 100% to 0% must be large. Consequently, the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in picture signal voltage .DELTA.V.sub.SIG must be made as small as possible. According to the foregoing driving method, however, the picture signal voltage V.sub.SIG is completely identical with the effective voltage V.sub.LC applied to the liquid crystal layer. Therefore, if the effective voltage applied to the liquid crystal layer is .DELTA.V.sub.LC, the following expression is obtained: EQU .DELTA.T/.DELTA.V.sub.SIG =.DELTA.T/.DELTA.V.sub.LC.
Since the range of the effective voltage V.sub.LC applied to the liquid crystal layer corresponding to where the light transmittance varies from 100% to 0% is generally as small as a few volts, it is difficult to make the foregoing ratio of .DELTA.T/.DELTA.V.sub.SIG small. Thus, the foregoing driving method could hardly realize a sufficient gradational display.